Megan Fox loves to lavish on-again/off-again boyfriend Brian Austin Green with expensive gifts — when she’s not yo-yoing on whether or not to accept his marriage proposal.

The 23-year-old Transformers star shelled out more than $70,000 on diamond-laced Rolex timepiece for Brian’s 36th birthday last month.

A spywitness blabs to Globe: “Megan often jokes that Brian is an ‘old man’ because he’s 13 years older so she bought him and a cane. They both had a good laugh over that. But then she handed him his real present — a gem-encrusted, specially made Rolex watch, which set her back $72,000.”

The spy adds, “Brian was floored — and really touched.”